An incandescent lamp is dimmed when it operates at a lower voltage than the nominal voltage for which it was designed. As an applied voltage decreases, lamp power and lumens also decrease accordingly. Conventionally, dimmers are solid-state switching devices that turn the light on and off at a rate that is related to the frequency of the applied power source. In the United States, standard alternating current (AC) power supply voltages oscillate at a rate of 60 times per second (or 60 Hertz), so the dimmer circuits are designed to turn the light on and off at a rate of twice the supply frequency (i.e., a frequency of 120 Hertz). The combined thermal mass and the resistance of the filament of the incandescent lamp smoothes out the pulses. Since the brightness at which the filament glows depends on the amount of current flowing through the filament, the human eye sees a brighter or dimmer light, depending on the proportion of on-time relative to off-time. In other words, the input voltage is turned on at a point in the phase, reducing the average voltage delivered to the bulb, which is called phase-cut dimming.
There are two types of phase-cut dimming: forward phase-cut dimming and reverse phase-cut dimming. In forward phase-cut dimming, a circuit controls the power supplied to the lamp so that the lamp is energized during the last portion of each power-line half cycle. Such forward phase-cut dimming circuits are inexpensive, relatively robust, and suitable for most load circuits, including regular incandescent lamps and magnetic transformers, as well as neon, cold cathode, and some types of fluorescent dimming ballasts. In reverse phase-cut dimming, a circuit controls power to the lamp so that the lamp is energized during the initial portion of each power-line half cycle. Reverse phase-cut dimming is typically more expensive because it uses more complex electronics. However, some loads, such as electronic transformers, operate better and make less audible noise (such as buzzing or ringing) when a reverse phase-cut dimmer circuit is used.
In either case, the actual lighting load operates at the power-line frequency. The switching frequency can cause the load circuitry to emit audible noise. Sometimes noise-reduction devices, such as de-buzzing coils, are used to reduce or eliminate the hum that these systems generate.
One type of phase-cut dimmer circuit is called a Triode for Alternating Current (TRIAC) circuit or TRIAC dimmer. TRIAC dimmers can be incorporated into light switches to provide dimming functionality. A TRIAC dimmer controls power to a load (such as the bulb) by gating the applied sinusoidal AC waveform from the current source based on either the forward or reverse type of dimming. In particular, the TRIAC dimmer turns on over a half-cycle of the AC power supply and then turns off at the end of each half-cycle, modulating the power delivered to the load. The on-off cycle of the TRIAC dimmer repeats during operation to drive the load, and the timing of the turn-on of the TRIAC dimmer controls the amount of energy supplied. In general, the TRIAC dimmer delivers a peak power supply to the load when the TRIAC dimmer is configured to turn on at the peak of the AC power supply waveform.
Such TRIAC dimmers are used in many residential, commercial, and industrial settings to provide adjustable lighting. Conventional TRIAC dimmers are designed to control power supplied to a resistive load, such as a standard incandescent or halogen-based lamp. Recent developments in light bulbs, such as compact florescent lamps and light-emitting diode lamps, provide enhanced efficiency and longevity; however, such devices do not necessarily operate as resistive loads from the perspective of a TRIAC dimmer. Accordingly, conventional TRIAC dimmer circuits may not function properly and/or may not work at all with such newer lighting solutions. Instead of dimming the lamp, such TRIAC dimmer circuits may cause the lamp to flicker or may not work at all.
In the following description, the use of the same reference numerals in different drawings indicates similar or identical items.